Marginal zone (MZ) B cells have been implicated in SLE-like autoimmunity in mice and thus may be important in the induction of SLE in man. This proposal is based on the hypothesis that several facets of MZ B cells (i.e. heightened levels of C3dg-binding CD21, their proximity and sensitivity to BAFF, and their proximity and possible sensitivity to Toll-like receptor (TLR)-binding DNA from micro-organisms) may augment the capacity of MZ B cells to respond to weak BCR stimuli. Importantly, the latter characterizes the interaction of autoreactive B cells with self antigen (Ag), and C3dg-coated apoptotic cells expressing intracellular molecules as surface Ag are likely prevalent within the MZ. The application's long-term objectives are to illuminate the mechanisms by which stimuli within the MZ environment and human MZ B cell surface receptor expression contribute to the enhanced activation of autoreactive B cells which are found within the MZ. Using isolated lgM+lgD-CD27+ MZ B cells and lgM+lgD+CD27- follicular (FO) B cells from human spleens, we will: (a) compare the efficacy of IL-4 and BAFF at promoting MZ and FO B cell viability and preventing BCR-triggered apoptosis, (b) compare MZ and FO B cells for density of receptors for BAFF and IL-4, (c) with a series of affinity-diverse anti-BCR:dextran conjugates CD21 binding sites, determine the degree to which BCR ligation with the CD21 :CD19 costimulatory complex (i) augments BAFF-dependent MZ and FO B cell proliferation under conditions of limiting BCR engagement and (ii) collaborates with BAFF and TLR9-binding CpG DNA motifs to enhance MZ or FO B cell differentiation, (d) explore the role of homotypic CD27:CD7O interactions in promoting the differentiation of MZ B cells upon activation by the above stimuli, and (e) examine whether autoreactive B cells in the MZ of normal individuals are triggered to secrete autoantibody when cultured with C3dg-coated apoptotic cells in the presence of BAFF and microbial DNA motifs. The above functional studies with defined in vitro stimuli should elucidate whether stimuli expected in the MZ environment, particularly during microbial infection, can collaborate in promoting the activation of human B cells under conditions of limiting BCR engagement. Such insights should suggest immunologic intervention therapies for blocking the activation of autoreactive B cells in SLE.